The present invention relates generally to web processing apparatus and, more particularly, to a speed control assembly for a web winder.
continuous web processing apparatus such as web printers generally include a web "unwind" or "supply" spool from which unprocessed web material is supplied and a web "rewind" or "windup" or "collection" spool upon which the processed web is collected. Each of these spools is typically mounted upon a separate, driven, winder apparatus which rotates the spool mounted thereon at a selected rate. As the diameter of the web wound about a spool changes, the rotational velocity of the spool must also change if a constant web supply or collection rate is to be maintained. The rotation rate of the web winding apparatus must also be adjusted to accommodate for speed fluctuations at various web processing operating stations which are positioned along the web between the unwind and rewind spools. The most common method for maintaining proper winder speed is through use of a dancer assembly. A dancer assembly is a device consisting of at least one idler roll which is positioned in contact with the web of material. The dancer roll is displaceable in a direction transverse to the direction of web movement and is biased in a direction which opposes the tension applied to the dancer roll by the web. The bias force is of a magnitude such that when the web processing machine is operating at its normal web tensions, the dancer is positioned near the center of its range of movement. If the speed of the web varies with respect to the speed of the associated winder in a manner which decreases web tension, the dancer is displaced by the bias force in a direction to take up the resulting "slack" in the web. If the operation of the associated winder with respect to web line speed is such that the tension in the web increases, the dancer is displaced by the web tension force in a direction which shortens the web path and reduces web tension.
Winder speed is controlled by varying the speed of the winder in response to the displacement of the associated dancer assembly.
Various methods of processing a dancer displacement signal to control winder speed are known in the prior art. One method, known as 100% proportional control, is illustrated in FIG. 1. In this method of control, the winder motor velocity is increased or decreased as a linear function of dancer displacement from a dancer position near one end of the dancer travel path. The winder velocity to dancer position relationship for a supply winder is indicated in solid lines. The winder velocity to dancer position relationship for a collection winder having a dancer assembly identical to that for the supply winder is indicated in dashed lines. In such a system, at one end of the dancer travel range the winder operates at full speed, and at the other end of the dancer travel range the winder stops. Typically, the range of dancer displacement is selected to be somewhat larger than the range of dancer displacement needed to compensate for changes in roll diameter in order to accommodate other transient fluctuations in web speed. Such 100% proportional speed control results in a system which is very responsive but difficult to stabilize. In situations where the web being processed is an extensible web such as plastic film, a 100% proportional control system becomes totally unstable and unusable.
In a variation of the 100% proportional control method illustrated in FIG. 1, the dancer displacement signal is used in the same manner to control web speed. However, it accounts for only a small portion, e.g. 10%, of the total winder velocity control signal. The remainder of the signal is a line speed reference signal produced by a web speed monitor positioned along the web at a point intermediate the unwind and rewind assemblies. In such a control scheme, the dancer is relatively unresponsive and thus the system is easy to stabilize for stead-state conditions. However, in such a scheme, the dancer typically runs off-center some amount to compensate for calibration error or web stretch. This type of system experiences trouble with major tension variations in the web and will reach a mechanical limit for correction due to the dancer's lack of responsiveness.
In another method of winder speed control, the winder is provided with a tachometer which provides a speed signal. This winder speed signal and a web line speed signal are provided to a computer and used to compute the associated winder spool web diameter. A base winder speed is then calculated by dividing line speed by winder spool web diameter. The calculated winder speed is thereafter trimmed with a velocity signal calculated as a linear function of dancer displacement such as illustrated in FIG. 1. Such systems are quite expensive and require factory technicians for accurate calibration and setup.
Another method of winder speed control is known in the art as 100% integrated dancer centering speed control. According to this method, an analog integrator receives an input representative of linear dancer displacement and a winder acceleration (as opposed to velocity) signal is calculated which is linearly proportional to the dancer displacement signal. In such a system, the dancer under normal operating conditions remains at the center of its displacement range. However, it is generally difficult to find a balance between stability and responsiveness for such a control system. A graph indicative of winder motor acceleration response to dancer displacement for such a system is illustrated in FIG. 2.
In a variation on the 100% integrated dancer centering control system illustrated in FIG. 2, a signal identical to that illustrated in FIG. 2 is initially provided. However, the control system rather than using this signal as a motor acceleration signal instead uses it as a spool diameter signal and the winder velocity signal is provided by dividing web line speed by this diameter signal. Such systems generally require a factory technical for setup. Such systems are subject to failure due to calibration shifts and also experience stability problems.